Exclusive: Isis, facing a fierce assault on Mosul, its last stronghold in Iraq, is beset by divisions, desertions and the fear that it has been infiltrated by Western intelligence agencies, foremost among them the British

"They are afraid of a lot of things now. They are afraid of the bombing, they are afraid of the attack that's coming and they are also really afraid of the foreign spies who are among them".

This was Rashid's description of what was going on inside Isis. And the Western intelligence agency that has infiltrated Isis the most, claimed the Belgian jihadi, was the British.

Rachid is one of the thousands of foreign fighters who had gone to join Syria's jihad, graduating in extremism among rebel groups to two years serving with Isis. But disillusioned, he says, with the Islamists and fearing what lay ahead, he has fled across the border to Turkey.

Rachid, a mechanic before he took up the gun in the name of Islam, was part of a small but steady flow of fighters leaving Isis as it faces an assault on Mosul, its last stronghold in Iraq and waited for the coming offensive against Raqqa, the capital of its "caliphate" in Syria.

The contact with Rachid came through an intermediary in Turkey after I wrote an article in The Independent last month about thousands of jihadis from who, with Isis facing defeat, will be seeking to return.

The picture painted by him was of an "Islamic State" where the sense of omnipotence that came during the astonishingly quick advance through Syria and Iraq has been replaced by foreboding, divisions and desertions.

And with it has come rising paranoia and violent retribution being exacted on those deemed to be traitors.

Rachid, who had trimmed his beard and burned the clothes and documents he had in Syria, was anxious to leave Urfa, the city where we met, as soon as possible. It has, despite being in Turkey, become a haven for Islamist extremists, including those from Isis.

Moderate activists have been murdered in Urfa, among them Ibrahim Abdul Qader and Fares Hamadi, who were members of the group Raqqa Is Being Slaughtered Silently, which charts the brutality of Isis.

A video posted by Isis of the killings claimed the victims had "conspired with crusaders". There is no evidence that any of these activists had links with British or other intelligence agencies.

Many of those killed across the border, however, have been accused of being foreign spies.

One of the most high-profile figures was Abu Ubaida Almaghribi, head of Isis's security for Aleppo, who was in charge of James Foley, the American photojournalist, and other Western hostages before they were murdered.

Almaghribi, a Dutch national of Moroccan descent, and three others were said to have been beheaded after being accused of being MI6 agents and passing on secrets to the UK.

Isis held an internal investigation after British national Mohammed Emwazi, who murdered James Foley and other captives, including Britons David Haines and Alan Henning, was killed in another drone strike last November.

A purge and executions followed claims that his location had been betrayed, enabling the successful air strike to be carried out.

Rachid and other European jihadis face the prospect of long prison sentences if they returned home. The only European state that does not charge returnees from Syria's rebel groups with terrorism related offences is Denmark, which runs a rehabilitation programme .

The foreign fighters were convinced, said Rachid and other fighters, that the Western security agencies are tracking them after placing agents inside Isis and other Islamist group.

They are also convinced that Britain has taken a leading role in this. "The British are not the biggest group of volunteers from a country [in Europe].

But maybe because some of those who came from there became so well known that their intelligence service is so involved.

But many brothers will tell you that spies are active, Daesh [Isis] have killed men, women even, after saying they were foreign spies, some of them British spies," said Rachid.

"Daesh asked for a lot of people, not just fighters, but teachers, engineers, doctors to build the Islamic State, if you remember. They now think that a few of these spies got in at that time," he added.

Uluk Ultas, a Turkish academic and political analyst who is writing a book on Syrian rebels was also aware of the "British connection".

He said: "There is a pretty widespread feeling that the UK has managed to infiltrate a number of groups, including Isis, and that the Isis leadership was aware of this and worried.

"I was surprised when I was first told this, because I thought it would be the Americans who would be taking the lead in this among the Western countries, but it seems the British have been working on this for a while, don't forget Almaghribi was arrested by Daesh two years ago."

Mr Ultas is the director of Seta, a think tank which was founded by Ibrahim Kalin, an adviser to President Recep Tayyep Erdogan. His book will contain a rare interview with Abu Firas al-Suri, a senior leader of Jabhat al-Nusra who was subsequently killed in a US drone strike.

He continued: "What is interesting is that as Daesh [Isis] got bigger the checks it made on those wishing to join became less strict.

"It seems the feeling in the leadership was that since they were no longer just a fighting group, but the Islamic State, they were somehow above it and that their security would find enemy agents.

"But it seems they were not so successful and Western intelligence and others like the Turkish and the Jordanian were able to get in there. Almaghribi was the most high profile of those Daesh called a British spy and said they killed.

"But I have spoken to fighters who say that Almaghribi isn't dead after all and has been seen around. Maybe he's being held as a bargaining counter for when Raqqa falls - who knows?"

Some of the information about Almaghribi came from Jejoen Bontick, who was, like Rachid, a Belgian jihadi. He had stated that James Foley and his fellow hostages were being held by Belgian jihadis who belonged to a group called Sharia4Belgium in Aleppo.

They were later moved to Raqqa and into the hands of Emwazi and three other Britons, who became known to their captives as The Beatles (Emwazi was known as "Jihadi John").

Philip Balboni, the chief executive of GlobalPost, the American news organisation for which Foley worked, had said of Bontick: "A young Belgian who had travelled to fight in Syria befriended Foley and, once that jihadist went back to Belgium, offered excellent information on roughly where Jim was held and by whom. It was the first time we knew that Jim was alive."

Bontinck was a member of Sharia-4-Belgium. It's leader Fouad Belkacem, a convicted burglar, visited England in 2010 and met Anjem Choudary, the London-based radical Muslim preacher for advice before starting the group in Brussels. Choudary was jailed for five and half years in London last month after being convicted of supporting Isis.

Some Sharia-4-Belgium members subsequently visited Choudary and worked with him in London: some of the preacher's followers went in turn to Belgium.

Choudary, who represented fundamentalist British Muslim groups Islam4UK and al-Muhajiroun, gave his support to Sharia4Belgium through a website called Paltalk.

It was also used by Omar Bakri Mohammed, another radical preacher who was expelled from the UK. After the murder of Lee Rigby, Bakri Mohammed told The Independent that one of the killers, Michael Adebolajo, used to be part of his congregation.

Sharia4Belgium founder Belkacem has been jailed for 12 years in Belgium, Boninck received a suspended sentence after testifying against others, many of them members of the organisation, accused of terrorist offences,

Around 27,000 foreign fighters are believed to have joined Isis since the start of Syria's civil war five years ago, with between 5,000 and 7,000 of them arriving from Europe to join Isis and other hardline Salafist groups such as Jabhat al-Nusra - an al-Qaeda affiliate that now calls itself Jabhat Fatah al-Sham. Just under 800 have travelled from the UK, but the numbers coming back have been fewer than a dozen for the past nine months.

"The British brothers are not going back because they are afraid of getting arrested like the others from Europe" said Rachid.

"But staying in Syria now means there is a big risk of being killed, either from the bombing, or when the attack on Raqaa starts or by Daesh, if they decide you are a traitor, a spy."